1. Field of the Invention
This invention relates to feedback control of electromagnetic actuators, and more particularly relates to feedback control of actuators for impact printing, by means of pulse-width modulation of the coil-energizing waveform, based on one or more measurements of state variables.
2. Prior Art
In impact printing, a mass is accelerated, typically by electromagnetic means, toward a rigid platen, with paper and inked ribbon intervening. Sometimes, as in engraved band line printers, a type carrying element also intervenes. In any case, ink is transferred to the paper at the instant when these various elements are compressed between the accelerating mass and the platen. The force of compression (the "impact force") is controlled by the speed of the accelerating mass just prior to impact (the "impact velocity"). Therefore, to produce high quality printing of uniform darkness, it is necessary to regulate the impact velocity: too low an impact velicty will cause the printed image to be too light, while too high an impact velocity may cause the paper to "punch through," and may also damage the ribbon.
In many impact printers, it is also necessary to regulate "flight time"--the interval which elapses between the beginning of an actuation and the impact, because there is lateral motion between the various elements involved in the impact. For example, in an engraved band line printer, the band moves laterally at high velocity with respect to the hammer, paper, ribbon and platen; in a serial wire matrix printer, the wire moves laterally with respect to the paper. Thus if flight time varies too much from the expected, nominal value, then the printed mark may be noticeably misplaced on the paper. In the case of engraved band printers, the character may also be truncated on one side or the other. In general, the required precision of flight time regulation increases in proportion to the speed of the lateral motion. Typically, the greatest precision is needed in engraved band line printers, where the lateral speed of the band is typically many meters per second; less precision is needed in serial wire matrix printers, where the lateral speed of the carriage is typically less than one meter per second.
In summary, the purpose of controlling an impact-printing actuator is to regulate the trajectory of the accelerating mass (hereafter called "the hammer" or "the armature") to achieve repeatable conditions at impact; in particular, repeatable flight time and impact velocity.
Many disturbances conspire to perturb a hammer's trajectory. For example, if the hammer's repetition rate is high, its position and velocity at the start of each actuation, nominally zero, may be perturbed by "settle-out"--residual vibrations left over from the previous firing. These vibrations are typically damped out, after the return stroke of the armature, by multiple, energy absorbing impacts against a backstop. However, if each actuation is forced to wait for the previous actuation to settle out, the maximum repetition rate of the actuator--and hence the speed of printing--may be unacceptably low. Even when the repetition rate is low, there are still perturbations caused by "mechanical interaction" (residual vibrations left over from previous firings of neighboring hammers); by "magnetic interaction" (stray magnetic flux generated by neighboring hammers); by fluctuations of the supply voltage which drives the coil of the electromagnet; by wear at the impact surfaces; by thermal variations of electrical and mechanical components; and by small variations in frictional forces which occur naturally as a consequence of environmental conditions and the accumulation of dirt.
A number of compensation and control schemes have been disclosed in the prior art to minimize or counteract these sources of perturbation, and thereby to provide faster printing having higher print quality. The following references, arranged chronologically, are representative:
1. Joseph P. Pawletko and Charles O. Ross, "Printer Hammer Compensation," U.S. Pat. No. 3,513,774, IBM Corp., July 1, 1968. (A3, 03, M2). PA0 2. Charles B. Pear Jr. and Joseph A. Ross, "Hammer Firing System for a High-Speed Printer," U.S. Pat. No. 3,678,847, Potter Instrument Co., July 25, 1972. (A2, 01, M1). PA0 3. Richard Lyman Gilbert and Michael David Hryck, "Printer Density Control, " U.S. Pat. No. 3,834,306, IBM Corp., Sept. 10, 1974. (A3, 02, 03, M2). PA0 4. Andrew B. Carson and Michael J. Tuzo, "Drive Circuit for Printing Head," U.S. Pat. No. 4,162,131, General Electric Co., July 24, 1979. (A2, 02, M2). PA0 5. Nico Blom and Jan T. Wor, "Printer, Provided with an Impact Device Comprising a Transducer," U.S. Pat. No. 4,192,230, U.S. Philips Corp., Mar. 11, 1980. (A1, A2, 02, 03, M4). PA0 6. Klaus Arendt, Werner Hasler, and Karl-Heinz Schaller, "Circuit Arrangement for Synchronizing the Times of Occurrence of the Print Hammer Impact with the Arrival of the Print Type at the Print Position, " U.S. Pat. No. 4,259,903, IBM Corp., Apr. 7, 1981. (A3, 03, M2). PA0 7. Hiroshige Nakano, Shigenobu Katagiri, Shinichi Nishino, "Magnetic Interference Prevention System," U.S. Pat. No. 4,278,021, Hitachi Koki Co, Ltd., July 14, 1981. (A3, 03, M2). PA0 8. Gordon B. Barrus and Jerry Matula, "Printer Having Variable Hammer Release Drive," U.S. Pat. No. 4,280,404, Printronix Inc., July 28, 1981. (A2, 02, M2). PA0 9. Mark H. Hoffman, "Printer Control System," U.S. Pat. No. 4,293,233, SCI Systems Inc., Oct. 6, 1981. (A1, 02, M2). PA0 10. Andrew B. Carson and Samuel C. Harris, Jr., "Driving Force Control System for Impact Printer," U.S. Pat. No. 4,333,398, General Electric Co., June 8, 1982. (A2, 01, 02, M4). PA0 11. Robert H. Sweat and William J. Thornhill, "Impact Printer Hammer Flight Time and Velocity Sensing Means," U.S. Pat. No. 4,347,786, IBM Corp., Sept. 7, 1982. (A1, 03, M3). PA0 12. Gordon Sohl, John R. Masters, and John R. Leicht, "Moving Coil, Multiple Energy Print Hammer System Including a Closed-Loop Servo," U.S. Pat. No. 4,353,656, Xerox Corp., Oct. 12, 1982. (A1, 01, 02, M4). PA0 13. David A. Hall and George P. Olson, "Solenoid Impact Print Hammer with Uniform Free Flight Time," U.S. Pat. No. 4,407,193, IBM Corp., Oct. 4, 1983. (A1, 03, M3). PA0 14. Ulrich Heider, "Impact Printing Device with an Improved Print Hammer," U.s. Pat. No. 4,429,342, Siemens Aktiengesellschaft, Jan. 31, 1984. (A1, A2, 01, 02, M4). PA0 15. Douglas A. Dayger, Michael D. Hryck, Dean W. Skinner and Gerald R. Westcott, "Control System for Timing Hammers of Impact Printers," U.S. Pat. No. 4,440,079, IBM Corp., Apr. 3, 1984. (A3, 03, M3). PA0 A1: Daisy Wheel Printer PA0 A2: Serial Wire-Matrix Printer PA0 A3: Engraved Band Printer PA0 o1: Minimize settle-out time PA0 O2: Regulate impact velocity (i.e. impact force) PA0 O3: Regulate flight time PA0 M1: Disturbance-free PA0 M2: Open-loop compensation PA0 M3: Closed-loop feedback on subsequent shots PA0 M4: Closed-loop, real-time feedback
The foregoing references have been classified in three ways: first by application, to describe the type(s) of printer considered; second by objective, to designate the purpose(s) of the control scheme; and third by methodology, to specify how disturbances are assessed and treated. Classifications appear in the above list at the end of each citation. The following code letters have been used: